Why do we say that?

The English language (and, for all I know, other languages as well) is full of rather peculiar phrases. For example, one of my schoolteachers used to threaten miscreants that he would have them "trotting along a sure as a couple of eggs". By "trotting along" he meant he would send them to the headmaster for punishment - possibly six of the best. I don't think the fact that said schoolmaster was Welsh had anything to do with the eggs, but I have often wondered what is so certain about a pair of eggs? And, come to that, why should six strokes of the cane be considered "of the best"? There are plenty of other examples of odd-sounding phrases that come to mind. "Raining cats and dogs" is one. I did once hear how that phrase came about but I thought it was a cock and bull story. See? There's another one! You want some more? How about "hell for leather", "like a bat out of hell" and "pull the other one"?

So how did these phrases come into existence? I have long intended buying a book explaining just that but for some reason have never actually done so - until now. Earlier this year I was given an Amazon gift voucher for my birthday. So far I have used it to buy the three volumes in the Loss of Eden trilogy (I suppose if it's a trilogy there would have to be three volumes, wouldn't there?) by John Masters. I paid only 1p each plus post and packing as they are second hand, the books being long out of print. I was left with quite a balance which I have used, in part, to buy Brewer's Dictionary of Phrase and Fable which I hope will supply the answers to many questions. I suppose I could look things up on the Internet (and why does the word "Internet" start with a capital letter?) but books are so much more conducive to these things. After all, it's good to be able to turn the pages, dipping into the book here and there as the whim takes.

I hope the book will arrive in a day or two. When it does, I should be able to tell why a couple of eggs are so sure and other unlikely tales.